Ignore unnecessary directories when linting

[kumar303]

When running web-ext lint in a source directory, the linter should ignore the artifacts directory and node_modules directory. Otherwise, it may begin parsing too many files which could lead to errors or unwanted linting results. Maybe there are other non-source directories we should ignore. We can probably re-use the filter rules from FileFilter.

See #396

[pdehaan]

Is there a way to specify what file(s) get added/excluded from the web extension using web-ext (similar to the .jpmignore file)?
Or is that basically the same issue/request as #176?

We have mozilla/blok#69 to audit the files in our generated ZIP/XPI, but I wasn't sure how to exclude things like our /docs/, /web-ext-artifacts/, /tests/, and /coverage/ folders which aren't needed in our web extension.

[kumar303]

We only have default ignore rules in place right now, which includes *.xpi and node_modules. It only applies to the build command currently. We should probably add options to the build command to customize it or, yes, a standard ignore file.